Simple steps to install ETS2 Mods:

- download the mod «FLIEGL ASS 2101 Agrar Trailer for Ets2»
- use WinRAR or 7-zip and unzip the archive;
- copy the file with the extension .scs in My Documents/Euro Truck Simulator/mod folder
- start the game, go to your profile, see the Mod manager and activate the mod.